The power output from a modern wind turbine can be controlled by means of a control system for regulating the pitch angle of the rotor blades. The rotor rotation speed and power output of the wind turbine can hereby be initially controlled e.g. before a transfer to a utility grid through power converting means. An advantage of this control is a protection of the rotor from rotating at an excessive speed at high wind speeds and save the rotor blades from excessive loads.
Especially for large rotor diameters, the distribution of the wind inflow profile can be non-uniform over the area of the rotor, resulting in a non-uniform load to each rotor blade as a function of one full rotation, as well as asymmetrical out of plane loadings for the drive train of the wind turbine. The asymmetrical load from the wind on the rotor plane results in the rotor being loaded with a varying tilt moment and a yaw moment. For a free wind inflow situation the wind shear distribution is approximately linear and the said load as a function of rotation is of nearly sinusoidal behavior with a frequency equal to the rotor rotation frequency. In order to keep a more constant load on the rotor blades, pitch control functions have been applied to wind turbine pitch controllers, where a rotor-cyclic correction with a frequency equal to the rotor rotation has been added to the overall pitch angle setting of the individual rotor blades.
Any obstacles within certain up wind distance of a wind turbine create a wake for the wind turbine and consequently eliminate the free wind inflow situation. An example of an obstacle may be other wind turbines, as a wind turbine always cast a wake in the downwind direction.
It is known in the technical area of wind turbines to provide means for pitch control means for the blades of the rotor, i.e. means for rotating each of the blades about the longitudinal axis to a predetermined angular position to obtain a pitch angle of the blade which may differ from the pitch angle of the other blade or blades of the wind turbine rotor, with the purpose of reducing the extreme loads as well as the load variations causing fatigue on the blades, in particular at the root of the blades, and on other parts of the rotor and transmission. The pitch control may be independent for each blade so that no dependency exists between the pitch angles of the individual blades, or the pitch control may be of the cyclic type, i.e. that the instantaneous pitch angle of a rotor blade depends on a function of the instantaneous azimuth angle of the rotor blade with the function being at least substantially the same for all blades of the rotor, whereby all blades go through substantially the same pitch angle sequence during a full rotation depending on the azimuth angle of the individual blade.
Caselitz et al, “Reduction of fatigue loads on wind energy converters by advanced control methods”, European Wind Energy Conference, October 1997, Dublin Castle, Ireland, discloses a method reducing the fatigue loads by analysing the asymmetrical load on the rotor to determine the aerodynamically induced tilt and yaw moments and compensate these by pitch of the rotor blades in order to reduce the varying loads on the blades and other parts of the wind turbine, in particular the flapwise bending moment variations, i.e. the moment bending the blades out from the plane of the rotor disc, on the blades causing fatigue.
E. A. Bossanyi “Individual Blade Pitch Control for Load Reduction”, Wind Energy 2003, Vol. 6, discusses the use of individual blade pitch control to reduce asymmetric aerodynamic loads due to wind speed variations across the rotor disc due to wind shear, tower shadow, yaw misalignment and turbulence. The blade load variations with a period of one revolution of a blade are analysed and the pitch of the individual blade is adjusted to compensate for the asymmetric loads, thus reducing the fatigue inducing load variations.
In EP 0 995 904 a wind turbine is disclosed having blades with adjustable angles of attack and a transducer which provides a measurement parameter giving a measure of the current load on an element of the wind turbine structure. The blade angle is adjusted depending on the measurement parameter, which represents an acceleration or deformation of the structural element and gives a measure of a force or torque. The rotor blades are individually adjustable and the adjustment is made in order to reduce impact forces on the wind turbine bearing and to reduce the variations in the loads so as to eliminate fatigue of the wind turbine elements. In WO 2004/074681 a similar method for controlling the aerodynamic load of a wind turbine is disclosed.
The size of wind turbine rotors and thereby the production from the wind turbines is steadily increasing, which also as a side effect induces increasing load on all parts of the wind turbine, in particular the drive train including blades, generator, bearings, a possible gear box etc. often resulting in a short lifetime for the involved components unless action is taken to reduce the loads that cause the shortening of the lifetime. The aerodynamic loads on the wind turbine parts may at least to some extent be controlled by a pitch control, and it is an object of the present invention to provide an improved control of the loads on the wind turbine part reducing their lifetime by means of operating the pitch control.